other than what's already been said, the MB200 also fits in the pocket of most gig bags. I carried one around for a couple years this way. I can't think of any smaller sized amp head with the parameters the OP needs/wants. Maybe a MB F1, but you're not talking $200 there, even for a good used one.

Just a thought. I would make sure your back up is rated for at least the same wattage/power as your main amp. Same to be said for the ohm loading. I carry a mb800 as a back up for my SVT CL head. Its light, packs easily in my gig bag and is always there to ward off the gremlins. I cant comment on the other amps, but gk's stuff is solid and is perfect for pushing those 810's if called upon in my situation. Ive played through the mb200 in a 212 cabinet and it sounded great. That would be my vote if that's the sound you are looking for.

I bought an Aguilar TH350 for just this purpose: as a 'backup' that actually fit in a gigbag or breifcase. I've ended up using it a LOT as my main amp though! Killer tone, overengineered (especially the power section) and so small and light!

A lot of good choices here. I've never been able to connect to the Genz Benz tone - have tried many of their heads and they just don't work for me. I have a Tonehammer 500 and its awesome so maybe the 350. The Carvin is interesting. The GK MB200 is something I had in the past and sold, maybe a little too hasty but its so tiny that its a candidate for sure. What is the absolute smallest micro head out there?